1. Field of the Invention
This invention relates generally to a junction for directing both satellite uplink and downlink signals, and, more particularly, to a coaxial turnstile junction for combining and directing satellite uplink and downlink signals where the junction has a taper in the wave launching section to provide impedance matching for waveguide irises.
2. Discussion of the Related Art
Various communications systems, such as certain cellular telephone systems, cable television systems, internet systems, military communications systems, etc., make use of satellites orbiting the Earth to transfer signals. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and then retransmitted by the satellite to another satellite or to the Earth as a downlink communications signal to cover a desirable reception area depending on the particular use. The uplink and downlink signals are typically transmitted at different frequency bandwidths. For example, the uplink communications signal may be transmitted at 30 GHz and the downlink communications signal may be transmitted at 20 GHz.
The satellite is equipped with an antenna system including a configuration of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth. Typically, the antenna system includes one or more arrays of feed horns, where each feed horn array includes an antenna reflector for collecting and directing the signals. In order to reduce weight and conserve satellite real estate, some satellite communications systems use the same antenna system and array of feed horns to receive the uplink signals and transmit the downlink signals. Combining satellite uplink signal reception and downlink signal transmission functions for a particular coverage area using a reflector antenna system requires specialized feed systems capable of supporting dual frequencies and providing dual polarization, and thus requires specialized feed system components. Also, the downlink signal, transmitted at high power (60-100 W) at the downlink bandwidth (18.3 GHz-20.2 GHz), requires low losses due to the cost/efficiency of generating the power and heat generated when losses are present.
These specialized feed system components include signal junctions, such as coaxial turnstile junctions, known to those skilled in the art, used in combination with each feed horn to provide signal combining and isolation to separate the uplink and downlink signals. The current turnstile junctions are limited in their ability to provide suitable impedance matching between the downlink waveguide and the junction over the complete downlink frequency bandwidth. Thus, there is a need to provide a junction that has better impedance matching between the junction and the downlink waveguides. It is therefore an object of the present invention to provide an improved coaxial turnstile junction for this purpose.
In accordance with the teachings of the present invention, a coaxial turnstile junction is disclosed for combining and directing both satellite uplink and downlink signals, that includes a tapered section to provide an improved impedance matching for the downlink signal between the junction and the downlink waveguides. The junction includes a first end that is in signal communication with an antenna feed horn. The first end of the junction includes a cylindrical outer wall and a cylindrical inner wall that are coaxial and define an outer chamber and an inner chamber. The outer wall extends into the tapered section at a second end opposite the first end, where the tapered section contacts the inner wall and closes the outer chamber at that end. A plurality of symmetrically disposed waveguides are positioned around the outer wall and are in signal communication with the outer chamber through openings in the tapered section. Irises are provided at the connection between the downlink waveguides and the outer chamber for impedance matching purposes.
Satellite downlink signals propagate through the downlink waveguides to the feed horn through the outer chamber. Satellite uplink signals received by the feed horn are directed through the inner chamber and exit the second end to be sent to receiver circuitry. The dimensions of the irises and the flare angle of the tapered section are selected and optimized so that the downlink signal from the downlink waveguides is impedance matched to the outer chamber at the downlink frequencies.